Heat a frying pan until hot, then add the butter and kidneys and fry for 2-3 minutes, until golden-brown all over.
Place the Worcestershire sauce, tomato purée, lemon juice, mustard powder and cayenne powder into a bowl and whisk together well.
Pour the dressing onto the kidneys and stir well to combine. Cook for a further minute or two, until the kidneys are completely cooked through. Season, to taste, with salt and freshly ground black pepper.
To serve, place the toast onto two plates and top each slice with the kidneys and pan juices. Garnish with flatleaf parsley.